Processes for the conversion of lower molecular weight alkanes such as methane to higher molecular weight hydrocarbons which have greater value are sought. One of the proposals for the conversion of lower molecular weight alkanes is by oxidative coupling. For instance, G. E. Keller and M. M. Bhasin disclose in Journal of Catalysis, Volume 73, pages 9 to 19 (1982) that methane can be converted to, e.g., ethylene. The publication by Keller, et al., has preceded the advent of substantial patent and open literature disclosures by numerous researchers pertaining to processes for the oxidative coupling of lower alkanes and catalysts for such processes.
In order for an oxidative coupling process to be commercially attractive, the process should be capable of providing a good rate of conversion of the lower alkanes with high selectivity to the sought higher molecular weight hydrocarbons.
Two general types of oxidative coupling processes are the sequential, or pulsed, processes and the cofeed processes. The sequential processes are characterized by alternately cycling an oxygen-containing gas and an alkane-containing gas to contact a catalyst. These processes typically provide high selectivities to higher hydrocarbon but suffer from operational complexities in cycling the catalyst environment and in the tendency of the processes to produce less desirable, higher molecular weight products and to have carbonaceous deposits form on the catalyst, thereby necessitating regeneration. Thus, from an operational standpoint, cofeed processes, i.e., processes in which oxygen-containing material and alkane are simultaneously fed to the reaction zone containing the catalyst, are more desirable.
In order for cofeed processes to be commercially attractive, especially for the production of large volume commodity chemicals such as ethylene and ethane (C.sub.2 's), the conversion of alkane should be high as well as the higher hydrocarbons as opposed to combustion products such as carbon dioxide and carbon monoxide.
Oxidative coupling occurs in the absence of catalysts, and this can occur in a homogeneous phase with oxygen and lower alkane being converted to carbon oxides (predominantly carbon monoxide) and higher hydrocarbons. The reaction proceeds at elevated temperatures, usually in excess of 600.degree. C. In order to enhance conversions of the lower alkane and selectivities to the higher hydrocarbons, catalysts have been used. Heterogeneous reactions are believed to occur on the surface of the catalysts.
Numerous researchers have studied the mechanisms by which oxidative coupling occurs. See, for instance, Campbell, et al., "Gas-Phase Coupling of Methyl Radicals during the Catalytic Partial Oxidation of Methane", J. Am. Chem. Soc., Vol. 109, 7900 (1987); Hutchings, et al., "The Role of Gas Phase Reaction in the Selective Oxidation of Methane", J. Chem. Soc., Chem. Comm., 253 (1988); Otsuka, et al., "Synthesis of C.sub.2 H.sub.4 by Partial Oxidation of CH.sub.4 over Transition Metal Oxides with Alkali Chlorides", Studies in Surface Science & Catalysis: #36 Methane Conversion Symp., Auckland, New Zealand (1987); Yates, et al., "Blank Reactor Corrections in Studies of the Oxidative Dehydrogenation of Methane", J. Catal., Vol. 111 (1988); Driscoll, et al., "Gas-Phase Radical Formation during the Reactions of Methane, Ethane, Ethylene and Propylene over Selected Oxide Catalysts", J. Phys. Chem., Vol. 89, 4415 (1985); Driscoll, et al. "The Production of Gas Phase Methyl Radicals over Li/MgO", in Che, et al., eds., Adsorption & Catalysis on Oxide Surfaces , Elsevier Science Publishers B. V., Amsterdam (1985); Hatano, et al., "Alkali Metal Doped Transition Metal Oxides Active for Oxidative Coupling of Methane", Inorg. Chim. Acta, Vol. 146, 243 (1988); Minachev, et al., "Oxidative Coupling of Methane", Russian Chem. Reviews, Vol. 57, 221 (1988); Shigapov, et al., "Peculiarities in Oxidative Conversion of Methane to C.sub.2 Compounds over CaO-CaCl.sub.2 Catalysts", React. Kinet. Catal. Lett., Vol. 37, 397 (1988); Burch, et al., "Comparative Study of Catalysts for the Oxidative Coupling of Methane", Appl. Catal., Vol. 43, 105 (1988); Martin, et al., "Oxidative Conversion of Methane and C.sub.2 Hydrocarbons on Oxides: Homogeneous versus Heterogeneous Processes," Appl. Catal., Vol. 47, 287 (1989); Lane, et al., "Methane Utilization by Oxidative Coupling,"J. Catal., Vol. 113, 114 (1988) and Asami, et al., "Vapor-Phase Oxidative Coupling of Methane under Pressure," Energy & Fuels, Vol. 2, 574 (1988).
A thread from the mechanisms postulated by these workers is that even in heterogeneous catalysts systems, homogeneous reactions occur in the vapor phase. Many propose that the catalyst serves to generate, e.g, methyl radicals which are then coupled in the vapor phase to produce ethane or ethylene (C.sub.2 's). Yates, et al., for instance, show at 700.degree. C., in an empty reactor 23 percent of the methane can be converted with a selectivity to C.sub.2 's (ethylene and ethane) of 20.9 percent. Hence, vapor phase oxidation of alkane can be material to the performance of a catalytic process. Hatano, et al., supra, opine that the stability of the solid solution of alkali metals with transition metal oxides under steady state reaction conditions is essential for the alkali-doped oxide to be effective in the oxidative coupling of methane. Shigapov, supra, postulates that the chloride radicals associated with, e.g., calcium chloride, are responsible for increasing selectivity.
Among the numerous catalysts which have been proposed by researchers for oxidative coupling processes include catalysts containing alkali and/or alkaline earth metals. The alkali and alkaline earth metals have been suggested as being in the oxide, carbonate and halide forms. Other components such as rhenium, tungsten, copper, bismuth, lead, tin, iron, nickel, zinc, indium, vanadium, palladium, platinum, iridium, uranium, osmism, rhodium, zirconium, titanium, lanthanum, aluminum, chromium, cobalt, beryllium, germanium, antimony, gallium, manganese, yttrium, cerium, praseodymium (and other rare earth oxides), scandium, molybdenum, thallium, thorium, cadmium, boron, among other components, have also been suggested for use in oxidative coupling catalysts. See, for instance,
U.S. Pat. Nos. 4,450,310; 4,443,646; 4,499,324; 4,443,645; 4,443,648; 4,172,810; 4,205,194; 4,239,658; 4,523,050; 4,442,647; 4,499,323; 4,443,644; 4,444,984; 4,659,668; 4,704,487, 4,777,313; 4,780,449; International Patent Publication WO 86/07351, European Patent applications nos. 189079 (1986); 206042 (1986); 206044 (1986), and 177327 (1985), Australian Patent No. 52925 (1986), Moriyama, et al., "Oxidative Dimerization of Methane Over Promoted MgO, Important Factors," Chem. Soc. Japan, Chem. Lett., 1165 (1986), and Emesh, et al., "Oxidative Coupling of Methane over the Oxides of Groups IIIA, IVA and VA Metals," J. Phys. Chem, Vol. 90, 4785 (1986).
In many instances, the literature and patents describing oxidative coupling processes and catalysts do not relate experiences with catalyst stability. However, due to the lifetime problems that have plagued oxidative coupling catalysts, skepticism appears to exist that oxidative coupling catalysts exhibit long useful lifetimes absent empirical demonstrations.
Several researchers have attributed deactivation in certain catalysts to the loss of volatile salts from the catalyst. Otsuka, et al., note in "Active and Selective Catalysts in Oxidative Coupling of Methane. Nickel Oxides with Salts of Alkali Metals," Inorg. Chim. Acta, Vol. 118, L23 (1986), that the loss of catalyst performance could be due to the alkali salts investigated not being stable under oxidative coupling reaction conditions. They suggest that the salts may decompose, evaporate, react with methane and oxygen, or produce mixed oxides with NiO. Otsuka, et al., in "Synthesis of Ethylene by Partial Oxidation of Methane over the Oxides of Transition Elements with LiCl." Chem. Soc. Japan, Chem. Lett., 903 (1986), states that lithium chloride was found deposited at the outlet of the reactor. Fujimoto, et al., in "Selective Oxidative Coupling of Methane Over Supported Alkaline Earth Metal Halide Catalysts,"Appl. Catal., Vol. 50, 223 (1989), state that
". . . the decrease in catalytic activity for C.sub.2 formation over the MgCl.sub.2 /CaO catalyst can be attributed to the loss of halide ion." ". . . by replenishing the trace amound of chloride from the vapor phase with chloroform no change in selectivity was observed . . ." (p.227, 228).
Several researchers have proposed the use of alkali or alkaline earth metals in the form of halides (e.g., chloride, bromide or iodide) in oxidative coupling catalysts. The addition of hydrogen halide, halogen and/or organic halide has also been proposed. Australian Patent No. 52925 discloses the use of supported calcium chloride, barium bromide, potassium iodide, lithium chloride, cesium chloride, among others, for catalysts to oxidatively couple methane to ethane and ethylene. The patentees disclose feeding hydrogen halide to the reaction zone. European Patent Application 210 383 (1986) discloses the addition of gas phase material containing halogen component such as chlorine, methyl chloride and methyl dichloride. Enhanced selectivities are reported when the halogen component is present. The catalysts include those containing one or more of alkali and alkaline earth metal, alkali metal with lanthanide oxide, zinc oxide, titanium oxide or zirconium oxide, and others. The applicants postulate that the halide is important to increase conversion, especially to unsaturated materials. U.S. Pat. No. 4,654,460 discloses the addition of a halogen-containing material either in the catalyst or via a feed gas in an oxidative coupling process. The catalyst contains one or more alkali metal and alkaline earth metal components. Although no working examples are provided, conversions of methane are said to be increased with halides and selectivities to higher hydrocarbons, particularly ethylene, occur. See also, Burch, et al., "Role of Chlorine in Improving Selectivity in the Oxidative Coupling of Methane to Ethylene," Appl. Catal., Vol. 46, 69 (1989), who propose mechanistic possibilities for the effect of halide in oxidation coupling of methane.
Burch, et al., in their 1989 article, supra, queried whether the instability of metal halide-containing catalysts could be remedied by the addition of gaseous chlorinated compounds such as dichloromethane. They also indicate that the influence of the chloride addition is relatively short-lived with the catalysts tested and therefore continuous addition of the chloride is suggested. From a mechanistic standpoint, the authors opine that chlorides inhibit total oxidation and promote dehydrogenation of ethane.
U.S. Pat. No. 4,544,784 discloses the activation of certain catalysts for oxidative coupling of methane. The patentees state that methane conversion can be improved by at least periodic introduction of a halogen compound and that the presence of at least one alkali metal prolongs the effect provided by the addition of the halogen compound. The halogen compounds proposed are hydrogen halides, ammonium halides, aromatic halides and halogen gas. (Column 6, line 45, et seq.).
Other reports of halide addition to processes for the oxidative coupling of methane include European Patent Application Publications Nos. 206,043, 216,948 and 198,251.